In conventional suspensions for use on construction machines, especially, hydropneumatic suspensions for wide use on off-road dump trucks and rough-terrain cranes, the spring constant of each of these suspension is determined by the amount of nitrogen gas, for example, sealed in the suspension. Therefore, the resonance frequency of the suspension is uniquely determined by a load to be sustained. Based on this, one of the spring constant for a flat road and the spring constant for a highly bumpy unprepared land is preset to each construction machine. Consequently, a construction machine set with the spring constant for a flat road cannot efficiently absorb the vibration of resonance frequency, hampering the riding comfort when traveling on a bumpy unprepared land, and vice versa.
To solve this problem, a suspension disclosed in Japanese Unexamined Patent Publication No. 1-56217 (published date: 03.03.89) was proposed by the applicant hereof. Disclosed in this publication is a suspension that is mounted on a motor scraper, for example, as shown in FIG. 6. To be more specific, between an up-chamber 56 and a down-chamber 55 of each one of the suspension cylinders 51, 51, a respective one of the spring constant change-over valves 61, 61 is arranged. These spring constant change-over valves 61, 61 are operated by a manual switch 63. Each one of the down-chambers 55 is connected to a respective one of the accumulators 58, 58 through a respective one of the leveling control valves 57, 57. Each one of the accumulators 58, 58 is supplied with pressurized oil, discharged from a pump 59 through a respective one of the leveling control valves 57, 57.
In such a construction, each one of the leveling control valves 57, 57 is set to a neutral position N in a normal traveling mode, to a first position I when the vehicle height is raised, to a third position III when the vehicle height is lowered, and to a second position II when each one of the suspension cylinders 51, 51 is locked. The spring constant of each one of the spring constant change-over valves 61, 61 at its first position A is determined by a pressure receiving area A0 in the down-chamber 55 of the respective one of the suspension cylinders 51, 51 and a gas pressure P0 of the respective one of the accumulators 58, 58. At the third position C, the spring constant is determined by a pressure receiving area A2, which is the difference between the pressure receiving area A0 of the respective down-chamber 55 and a pressure receiving area A1 of the respective up-chamber 56, and the gas pressure P0 of the respective one of the accumulators 58, 58. At the second position B, a pressure difference is caused by a throttle 60, so that the valve of the spring constant is between the valves of the two above-mentioned spring constants.
However, because the change-over operation of each of the spring constant change-over valves 61, 61 is performed by the manual switch 63, an inappropriate operation of this manual switch 63 may select an inappropriate spring constant, thereby deteriorating the riding comfort. Moreover, if the manual switch 63 is operated when there is a difference in pressure between the left and right suspensions at the time of steering, for example, a difference in length is caused between the suspension cylinders 51, 51, thereby lowering the vehicle steerability or stability.